Disclosed in embodiments herein are methods and systems for generation of separate gray-scale watermarked images having watermarks that can be retrieved, or viewed, using the same public key rotated to different orientation angles.
The basics of phase-shift based digital watermarks, or correlation-marks, are described in U.S. Pat. No. 6,252,971 for “Digital watermarking using phase-shift stoclustic screens,” by S. Wang, previously incorporated herein by reference. Briefly, if two similar cluster halftone patterns are superimposed on each other, the output appearances can differ significantly depending on the relative positions, or the phase shift, of the two patterns. For example, the two checkerboard patterns, 100 depicted in FIGS. 1A and 110 depicted in FIG. 1B, are essentially the same, except that the pattern 110 in FIG. 1B is a shifted version of the pattern 110 in FIG. 1A with an exactly “one-box width” shift. If the two patterns 100 and 110 are superimposed on each other with a perfect alignment, the result would be a completely black image as depicted by 120 in FIG. 1D. On the other hand, overlapping pattern 100 of FIG. 1A with itself, which can be considered another version of 100 with a zero-shift, gives an identical pattern to the original pattern 100, as depicted by 130 in FIG. 1C.
Referring now to FIGS. 2A-2C, a halftone pattern is shown at 200 having only it's central portion, shown as region 210, shifted in this manner. When the reference, or “public key”, represented by the halftone pattern 240 of FIG. 2B is overlaid on top of the pattern 200, the result is clearly visible as a black central region shown at 250 in FIG. 2C. The example depicted in FIGS. 2A-2C is a simple demonstration for the phase-shift digital watermark technique. The shifted central part 210 in the picture may be considered as a square watermark, which is retrieved as a black square 250 in the overlay shown in FIG. 2C. The shift required for an optimal retrieval is equal to a half period of the halftone structure, or π, in a general mathematic term. The problem with a simple “insertion”, however, is that the boundaries between the shifted portion 210 and the balance of the image are quite visible as a seam 220 shown in FIG. 2A. To hide the seam 220, the phase jump from zero to π should be replaced by a smooth phase transition.
Prior patents, such as U.S. Pat. No. 6,252,971 for “Digital watermarking using phase-shift stoclustic screens,” by S. Wang, hereby incorporated herein by reference in its entirety, describe a method to embed correlation-based phase-shift digital watermarks, also referred to a correlation marks, into halftone screens. By overlaying a transparency on the prints generated by the special halftone screen, for example as a public key, an invisible watermark embedded in the image can be retrieved.
An aspect of the disclosed system and method provided extends use and formation of correlation marks by enabling separate watermarked images to be formed by embedding a phase shifted watermark into an image by halftoning the image with the three-dimensional threshold array where at least one input thereto is a phase shift value using a halftone structure having a plurality of different, arbitrary Rotation Angles. The same public key, in the form of a transparency, can be used to retrieve the watermark from all of the images when overlaid atop each image at an Orientation Angle matching the Rotation Angle used for embedding the watermark in the image.
Disclosed in embodiments herein is a method for digital watermarking of a grayscale image, including receiving the grayscale image to be watermarked, determining the grayscale watermark to be embedded in the image, and creating a plurality of digital watermarked grayscale images by embedding a digital watermark into the image by halftoning the image with a three-dimensional threshold array where at least one input thereto is a phase shift value using a halftone structure defined by the spatial vectors Va(xa, ya) and Vb(xb, yb) having the same interior angle α (the angle between the two vectors Va and Vb), the same vector amplitude D, and a different Rotation Angle for each.
Disclosed in embodiments herein is a method for retrieving a watermark image from a plurality of digitally watermarked grayscale images, each formed by halftoning an image with a halftone structure having a different arbitrary Rotation Angle using a three-dimensional threshold array having phase shift value as an input, including overlaying a single public key transparency of a checkerboard pattern having halftone frequencies and angles matching the halftone structure used for forming the plurality of digital watermarked grayscale images atop each of the watermarked images and orienting the key with respect to the watermarked images at Orientation Angles matching the Rotation Angles used for embedding the watermark image into the corresponding digitally watermarked image to retrieve the watermark image.
Also disclosed in embodiments herein is a system for producing digital watermarked images including an input image source, image memory for storing the input image to be watermarked, watermark memory for storing the watermark to be embedded in the image, and an image processor including a three-dimensional threshold array where at least one input thereto is a phase shift value for embedding an invisible digital watermark into the image by halftoning the image with the three-dimensional threshold array using a plurality of halftone structures each defined by the spatial vectors Va(xa, ya) and Vb(xb, yb) having the same interior angle α, the same vector amplitude D, and a different Rotation Angle to produce a plurality of digitally watermarked output images.
The various embodiments described herein are not intended to limit the invention to those embodiments described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.